That 3 week mark seems to be a good point to pick out keepers too. For some reason at that age they seem to hit a stage where they resemble what they will look like as adults. That doesn't mean they will stay that way, but a "bad" baby at 3 weeks is not likely to get better as an adult, where a "superb" baby at 3 weeks is likely to at least be "good" as an adult. Before then, you can weed out the obvious ones, such as those who are scrawny or weedy, pinched noses, narrow shoulders, etc. After the 3 weeks mark they go into those awkward adolescent stages and don't start evening out again until 8 weeks and beyond.

Learning what to look for just takes practices. Look at lots and lots of animals and get input on what you are looking at. Ask experienced people to show you pictures and point out the good and bad that can be seen in that picture. Pictures aren't the end all of what the animal looks like (a long head can be corrected in a photo when angled slightly, for example), but it can still help you learn what to look for.